The present invention relates to layered catalysts having increased catalytic activity and a method for making them. In particular, the invention relates to metal chalcogenide catalysts.
The structural nature of the layered transition metal dichalcogenides is intimately related to their practical uses. The layered transition metal dichalcogenides have applications in areas as diverse as lubrication (B. R. Stupp, Thin Solid Films 84, 257 1981), catalysis (M. Boudart, R. A. Dalla, K. Feger, D. G. Loffer, M. G. Samant, Science, 228,717, 1985) and electrochemistry (H. trbutsch, Faraday Discuss. Chem. Soc. 70, 190, 1981). Thus, the lubricity of MoS.sub.2 stems from the weak bonding between adjacent basal planes. The intercalative properties of TiS.sub.2, which has been exploited in nonaqueous battery applications are likewise the consequence of the weak interlayer forces, Whittingham, M. S., Science 192, 1126 (1976).
Another important application of transition metal dichalcogenides occurs in the area of heterogeneous catalysis. MoS.sub.2 and, to a lesser extent WS.sub.2 are the active components in catalysts currently in largescale use for the removal of sulfur and nitrogen from a variety of petroleum feedstocks. It is widely believed (see, e.g., S. J. Tauster, T. A. Pecoraro, and R. R. Chianelli, J. Catal. 63, 515 (1980)) that the catalytically active sites are located at the edges of the crystal planes in this two-dimensional material. Usefulness of these materials results from their highly anisotropic structure, in which tightly bound two-dimensional layers are held together by weak van der Waals forces between layers. Single crystals of these materials have two types of surfaces that have very different characteristics. Basal plan surfaces are parallel to the layers; edge surfaces are perpendicular. The basal surfaces contain sulfur atoms that are bound to three metal atoms. The sulfur atoms provide physical adsorption sites for intercalates but are otherwise chemically inert. On the other hand, the edge surfaces can be formed only by breaking bonds within the layer and have been shown to be chemically and catalytically active. The structure of these edge surfaces, which lies at the heart of the chemical properties of these materials, is poorly understood at present.
The catalytic properties of these materials are usually observed with highly disordered powders, instead of with either crystalline or microcrystalline materials. This is because conventional crystal growth techniques yield materials with relatively low edge area because growth occurs primarily in the direction parallel to the layers. A well-ordered edge surface is difficult to create by cutting or polishing because the layers fold and break irregularly. The present invention is a new way of preparing chemically reactive surfaces with lithographic fabrication methods. Single crystals of MoS.sub.2 prepared in this way have a surface that consists primarily of edge planes, which allows exceptional control of the surface morphology.